Nonlinear optical transmission in materials has several applications including laser mode-locking, pulse shaping, optical
bistability, optical switching, and optical power limiting. Organic molecules suitable for functionalization have been
extensively investigated for their third order nonlinearities. We have measured the optical nonlinearity of different novel
organic and composite systems including nanocomposite polymer films of Au, Ag and Pt, Organic ionic crystals
(pyridinium and quinolinium salts), Au-alkanethiol clusters, thiophene based polymers, and Schiff base complexes, using
the z-scan and degenerate four wave mixing techniques, employing laser pulses of nanosecond and femtosecond
durations respectively. Most of these materials are found to be efficient optical power limiters under our excitation
conditions, and their nonlinear extinction coefficients have been calculated. Enhancement in the optical nonlinearity was
sometimes obtained even by mixing of two organic media. From degenerate four wave mixing experiments, the third
order nonlinear susceptibility χ(3) and figure of merit χ(3)/α values also have been determined for some of these
materials. The above experiments conducted in a large number of organic and composite materials unravel the potential
of these media for diverse photonic applications.